The present invention relates to a metal insert, in particular a threaded insert, adapted to be embedded into a receiving bore of a plastic part.
Such metal inserts generally are inserted into the receiving bore of the plastic part by pressing the insert into the plastic material softened by heat or ultrasonic energy (heat embedding, ultrasonic welding). Furthermore, it has become known to introduce the insert into the material of the plastic part by cold pressing or by injection molding. In order to prevent the insert from being withdrawn and from being rotated, the insert generally has its peripheral surface provided with surface irregularities such as undercuts, grooves, notches, indentations, toothings and the like. For example U.S. Pat. No. 4,046,181 discloses a sleeve-like insert having a peripheral surface comprising axial stepped conical portions which are provided with teeth and notches.
While such measures provide for increased resistance to withdrawal and rotation of the insert, it also increases to the insert's resistance to being inserted into the plastic part. Apparently, the requirements for maximal withdrawal and rotation resistance is somewhat in contrast to the requirements for minimal insertion resistance.